Conductive transparent layers today have many applications in display-devices, optoelectronics and as architectural glass. For these applications it is desirable, on the one hand, that transmittance in the visible part of the spectrum is as high as possible, and on the other hand, that conductivity is as high as possible or surface resistivity as low as possible. As a measure of the quality of conductive transparent layers, the Haacke quality factor ΦTC=T10/Rs can be used, which is defined in the Journal of Applied Physics, Vol. 47, pages 4086-4089 (1976). “T” stands for the optical transmittance of the layer (as a fraction of the incident radiation) and Rs for the surface resistivity in sq. For example, a layer with a transmittance of 90% and a surface resistivity of 3sq has a Haacke quality factor of 0.116−1. A layer with a transmittance of 80% and a surface resistivity of 5sq has a quality factor of 0.021−1.
Another important property of such a layer system is its etching performance. This depends on its chemical composition and its thickness. For a short etching time and good edge sharpness, it is important that the layer is as thin as possible, i.e. less than 100 nm.
To obtain high quality factors, it is beneficial to combine layer systems, of oxidic and metallic layers. For example, the interposition of very thin layers of silver between thin oxide layers is known. As a result of being sandwiched between. oxide layers, the silver layer is stabilized and protected on the one hand, while on the other hand, its reflection is reduced and transmittance thus increased. These layer combinations have the additional advantage of a low overall layer thickness, namely 100 nm or less. A layer system of indium tin oxide, by comparison, which is of comparable resistivity, is more than 500 nm thick (S. H. Shin and co-authors, Thin Solid Films 341 (1999) 225-229). Consequently, etching processes of the kind typically used for manufacturing displays are faster and the extent of undercut is less.
Layer systems of this kind are described, for example, in: DP 0 599 071 A1, JP 10062602 A and in the article by K. K. Choi and co-authors, Thin Solid Films 341 (1999) 152-155.
In the EP 0 599 071 A1 a layer system is described with the layer sequence indium tin oxide, silver or various silver alloys, indium tin oxide. By annealing at 300° C. for an hour, layers can be produced with a surface resistivity of 3.2sq and, at the same time, good transmittance in the visual part of the spectrum. For the wavelengths 435, 545 and 610 nm, a mean Haacke quality factor of 0.066 is obtained. The subsequent temperature treatment necessary for display applications is, however, a disadvantage, since it entails an additional. step.
In the JP 10062602 A a similar layer system is describe. Here, a thin silver layer containing at least 1.5 atomic percentage added gold is embedded between oxide layers consisting of tin oxide, indium oxide and small additions of other oxides. Layers are obtained which have a surface resistivity of 4-20sq and high transparency at 550 nm. The higher costs incurred due to addition of gold and the relatively high surface resistivity must be seen as disadvantages.
In Thin Solid Films 341, K. K. Choi and co-authors describe a layer system consisting of indium tin oxide followed by a silver layer and, as covering layer, indium tin oxide again. To improve conductivity, the indium tin oxide layers are deposited at 200° C., but the silver layer at room temperature. However, as a result of heating prior to deposition of the second layer of indium tin oxide, the optical transmittance and the electrical conductivity of the silver layer are impaired. In the best of cases, layers with a surface resistivity of 4sq and 90% transmittance at 550 nm are obtained.
It is also known that through selective choice of materials and coating parameters, transparent, conductive layer systems can be produced which have a resistivity of 2.93sq, transmittance values (measured against air) of 89.2% at 435 nm, 92.4% at 545 nm and 82.2% at 610 nm, and an overall layer thickness of 86.5 nm. For the three wavelengths quoted, this transparent conductor has a mean Haacke quality factor of 0.104 ohm−1.
In the field of display devices, transparent electrodes with even lower surface resistivity combined with high transmittance in the visible part of the spectrum, i.e. a high Haacke quality factor, are needed for large-area flat LCD displays and for computer monitors with screen diagonals preferably over 17″. This is necessary because of the image size, the high resolution and pixel count, as well as the speed of these displays. These requirements can no longer be met by the methods known to date.